LeetCode 129: Sum Root to Leaf Numbers

Given a binary tree containing digits from 0-9 only, each root-to-leaf path could represent a number.
An example is the root-to-leaf path 1->2->3 which represents the number 123.
Find the total sum of all root-to-leaf numbers.
For example,

    1
   / \
  2   3

The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.
Return the sum = 12 + 13 = 25.

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int sumNumbers(TreeNode root) {
        // Adopt depth first search
        return dfs(root, 0);
    }
    
    private int dfs(TreeNode node, int sum)
    {
        if (node == null)
            return 0;
            
        sum = sum*10 + node.val;
        
        // If encounter leaf node, return the calculated result.
        if (node.left==null && node.right==null)
            return sum;
            
        return dfs(node.left, sum) + dfs(node.right, sum);
    }
}